I have four plants of coreopsis Zagreb and last year some parts had brown stems. They now are one big bunch. A part near the right middle did not come up yet the other areas are coming up nicely. I don't use mulch. Can I plant another coreopsis Zagreb in that spot if I dig out the old dirt and put in new? Thank you.
Your problem does sound like a fungus. Coreopsis is relatively problem-free, but it does like a dry soil, even poor soil, and a sunny spot. So maybe the site where you put your plants is too moist for their comfort. We have had a very wet year, so that could also be part of the problem. If you want to plant more, consider adding soil to raise the site a bit and/or digging around it to allow better drainage.